The present invention relates to the production of stabilized aqueous dispersions of silica fume, suitable for greater economy and ease in transportation and greater possibility of utilization in industry.
During the commercial production of silicon and ferrosilicon alloys, the main by-product is the so-called silica fume. This consists of over 90% silicon dioxide of very chemically active and extremely fine particle size. Previously this was allowed to escape into the atmosphere. With the more recent requirements for pollution abatement the silica fume is now collected and disposed of. Up to 500 tons per week of silica fume is thus collected at one plant and carted off to a land fill.
The powdered silica fume is very difficult to handle, transport and use, mainly because of its very bulky, fluffy nature. Thus, for example, closed, air-tight trucks are required to transport this material. We have now developed a method of producing highly concentrated aqueous dispersions of silica fume, so that its transportation and ultimate utilization may be achieved at less cost.
The process of producing metallic silicon consists of feeding a charge of quartz, coal, and wood chips to the top of a tall furnace provided with carbon electrodes. At the high temperature at the bottom of the furnace, the silicon dioxide is reduced by the carbon to silicon, which melts, and is tapped periodically. To produce the ferrosilicon alloys, scrap iron is also added to the charge.
The gaseous by-products are carbon monoxide which oxidizes to carbon dioxide, and silicon suboxide which oxidizes to silicon dioxide, when the gases reach the top of the furnace. The particulates are the so-called silica fume.
Typical analyses and properties of the submicron silica fume are:
SiO: 96.09% PA0 Fe.sub.2 O.sub.3 : 0.34% PA0 MnO: 0.09% PA0 Al.sub.2 O.sub.3 : 0.21% PA0 CaO: 0.35% PA0 MgO: 0.23% PA0 K.sub.2 O: 0.59% PA0 Na.sub.2 O: 0.07% PA0 SO.sub.3 : 0.35% PA0 loss in ignition: 1.68% PA0 Surface area: 25.9 m.sup.2 /gram PA0 Particle size: 0.25-0.02 microns PA0 Average particle size: 0.12 microns PA0 Bulk density as generated: 4-6 pcf PA0 Bulk density (packed): 12-14 pcf PA0 pH: 6.7